This invention relates to machines for installing ground anchors such as are used to stabilize utility poles, and in particular to a torque controller for the anchor installer.
Ground anchors are typically driven into the ground by a hydraulically driven motor. The motor rotates a Kelly bar and hardware connectors which engage the anchor. As the Kelly bar and hardware connectors are rotated by the hydraulic motor, the anchor is driven into the ground.
The utility personnel who typically install anchors often do not understand the torque to pull-out capacity relationship and thus many anchors are installed incorrectly. These incorrect installations can pull out of the earth at reduced load levels and present a safety hazard or can cause electrical lines to fall to the ground. Standards for these installations are usually known to the utility standards engineer, but installers do not consistently achieve the installed torque. This is due primarily to the unavailability of suitable torque devices or because the torque devices available are too difficult to use. For example, a shear pin is often used. The shear pins, however, will be subjected to forces during use which will weaken the pins. The use of shear pins thus requires constant changing of pins during an installation procedure.
In addition to the known need for a minimum torque on a given anchor installation for pull-out capacity, a problem exists when the torque exceeds the manufacturers torque design limit for the anchor being installed. Such high torque installations frequently break the anchor or the drive mechanisms during installation, creating the problem of unknowingly leaving a partially broken anchor in the ground, creating a pull-out safety risk. Further, the anchors are often capable of withstanding more force than the driving tool. For example, whereas an anchor can withstand up to 15,000 to 18,000 foot-pounds of torsional force when driven into the ground, the tools that are used to drive the anchor into the ground (e.g. an installing wrench tube) may only withstand 10,000 foot-pounds. When the anchor is installed, the installer may check the anchor for its rating, but often does not check other components of the drive assembly. Thus, the other components can be driven beyond their capabilities. The driving tools may thus break, shear off connecting bolts, and send parts flying. These flying parts form projectiles which may cause injury.
The hydraulic motors used are ever increasing in their torque capacity. While high torque is needed for the hole auguring function of the motor, the torque output by the motor may need to be limited for other applications.
There are devices available which measure the torque but do not provide a means of controlling the maximum torque during installation. For example, the shear pin can limit torque, but, as noted above, it is difficult to use and is limited in its mechanical capacity. Further, the shear pin controls only the torque applied to the anchor and does not control the torque output by the motor to reduce the torque when the torque exceeds a desired value. Shear pins are also known to have maintenance problems during sustained use, a problem which tends to discourage its use.